functor by default

[CarloLucibello]

Makes everything a functor by default, avoiding the need to sprinkle

@functor T

everywhere in Flux's layers and similar use cases.
The types already decorated with @functor T or @functor T (a, b) won't be affected by the change.

The amount of breakage and unintended consequence this PR could produce is something I cannot estimate at the moment.

Fix #49

TODO:

• docs
• run Optimisers.jl tests on this PR
• run Flux.jl tests on this PR

[mcabbott]

The fact that there's still a method for AbstractArray{<:Number} means that it doesn't recurse into the reshape here, which is good I think:

julia> pr(x) = (@show typeof(x); x);

julia> fmap(pr, rand(3)');
typeof(x) = Vector{Float64}

julia> fmap(pr, reshape(rand(Int8, 4)',2,2))
typeof(x) = Base.ReshapedArray{Int8, 2, Adjoint{Int8, Vector{Int8}}, Tuple{}}
2×2 reshape(adjoint(::Vector{Int8}), 2, 2) with eltype Int8:
   53  -63
 -125  -58

The default functor doesn't seem able to reconstruct closures like:

julia> D = let W = rand(2,2), b = zeros(2)
             x -> tanh.(W*x .+ b)
           end
#11 (generic function with 1 method)

julia> fmap(pr, D)
typeof(x) = Matrix{Float64}
typeof(x) = Vector{Float64}
ERROR: MethodError: no method matching var"#11#12"(::Matrix{Float64}, ::Vector{Float64})
Stacktrace:
 [1] (::Functors.var"#3#6"{UnionAll})(y::NamedTuple{(:W, :b), Tuple{Matrix{Float64}, Vector{Float64}}})
   @ Functors ~/.julia/packages/Functors/1AaAn/src/functor.jl:8
 [2] (::Functors.DefaultWalk)(::Function, ::Function)
   @ Functors ~/.julia/packages/Functors/1AaAn/src/walks.jl:56
...

julia> fieldnames(var"#11#12")
(:W, :b)

julia> methods(var"#11#12")
# 0 methods for type constructor

In global scope, example from https://fluxml.ai/Flux.jl/stable/models/basics/#Building-Simple-Models just does nothing instead, unsurprisingly:

julia> W = rand(2, 5);
julia> b = rand(2);
julia> predict(x) = W*x .+ b;

julia> fmap(pr, predict)
typeof(x) = typeof(predict)
predict (generic function with 1 method)

julia> fieldnames(typeof(predict))
()

[mcabbott]

I think this will be slow. In FluxML/Flux.jl#1932 it needed a generated function to be as quick as before:

https://github.com/FluxML/Flux.jl/pull/1932/files#diff-34ea0dd342eeb0864015f5ab93bd9db67f6f46ec46d4bf8dbebe9b8c1a433b04R129-R141

[CarloLucibello]

let's keep this in mind for a future optimization-oriented PR

[CarloLucibello]

@mcabbott I'm not familiar with generated functions. Can you show me the snippet we should use here? Otherwise it will be done in a future PR.

[mcabbott]

My link above is a few lines off. I don't remember all the details, but I think my comments are saying that it works around Base.typename(T).wrapper taking 2μs every time. However, this PR doesn't do that, it calls S = constructorof(T). Maybe that removes the need for @generated. Might be worth timing these two?

[mcabbott]

Trying to time this today...

julia> struct Mine1{T,S}
       x::T
       y::S
       end

julia> x = Mine1([1,2,3], [4,5,6]')
Mine1{Vector{Int64}, Adjoint{Int64, Vector{Int64}}}([1, 2, 3], adjoint([4, 5, 6]))

julia> @btime Base.typename(typeof($x)).wrapper
  1.458 ns (0 allocations: 0 bytes)
Mine1

julia> @btime ConstructionBase.constructorof(typeof($x))
  1.458 ns (0 allocations: 0 bytes)
Mine1

julia> VERSION
v"1.12.0-DEV.1375"

and on Julia 1.6 -- slower but far from 2μs, even if I have a faster computer than I did then...

julia> @btime Base.typename(typeof($x)).wrapper
  14.194 ns (0 allocations: 0 bytes)
Mine1

julia> @btime ConstructionBase.constructorof(typeof($x))
  0.001 ns (0 allocations: 0 bytes)
Mine1

julia> VERSION
v"1.6.0"

[mcabbott]

I think Kyle said he had a branch doing something like this, using ConstructionBase. That appears to be able to reconstruct closures which is neat:

julia> adder = let y = ones(1)
                 x -> x .+ y
               end
#38 (generic function with 1 method)

julia> adder.y
1-element Vector{Float64}:
 1.0

julia> adder(2)
1-element Vector{Float64}:
 3.0

julia> using ConstructionBase

julia> newadder = constructorof(typeof(adder))([4 5 6])
#38 (generic function with 1 method)

julia> newadder(2)
1×3 Matrix{Int64}:
 6  7  8

It is happy to re-build things like reshape(rand(Int8, 4)',2,2), not sure how that will interact with fmap(f, x, dx) for gradients etc.

[ToucheSir]

I learned recently that it's possible to de- and reconstruct closure types. See JuliaGPU/Adapt.jl#58 for an implementation of this. Even if we can't functor all user-defined types by default, maybe this is something we could in a backwards-compatible way? I could see it as a pilot project of sorts too.

[CarloLucibello]

With Flux v0.15 in preparation I think this is the right time to merge this.

[CarloLucibello]

I marked as leaves

• Number
• AbstractArray{<:Number}
• AbstractString

Other suggestions?

[mcabbott]

Would be good for docs to note that not every AbstractArray of Numbers is a leaf, e.g. Transpose

[CarloLucibello]

If I can get an approval I would go on and merge this

[CarloLucibello]

I added some benchmarks. It seems that we have some regressions with respect to master, although not on flux types (I don't know why). I'm not sure how much we should care about this performance regression since the absolute values seem reasonable. If we do care we can try another implementation with @generated, cf #51 (comment), but not in this PR.

	cl/fun	master	cl/fun/master
fmap/concrete struct	0.458 ± 0.001 μs	0.125 ± 0.041 μs	3.66
fmap/flux dense	0.5 ± 0 μs	0.5 ± 0.042 μs	1
fmap/flux dense chain	1.46 ± 0.042 μs	1.42 ± 0.042 μs	1.03
fmap/named tuple	0.292 ± 0.042 μs	0.334 ± 0.042 μs	0.874
fmap/non-concrete struct	0.542 ± 0.041 μs	0.125 ± 0 μs	4.34
time_to_load	0.059 ± 0.0021 s	0.0518 ± 0.0054 s	1.14

[mcabbott]

Can this update the readme too? I think that (besides deleting @functor Foo) it wants a big notice that Functors@0.5 is sort-of opt-out now, defaults to calling ConstructionBase.

[mcabbott]

Let's do it.

[CarloLucibello]

let's unleash havoc on the world!
ahah, hopefully not